The flat pelvis is marked by a shortening of the anteroposterior diameter of the inlet. It looks as if it had been pressed together from before backward while in a soft condition. These and other deformities will be recognized in advance of labor by the routine application of the pelvimeter. The value of this instrument is so great, that no competent man does obstetrical work at the present time without using the pelvimeter as a routine. The average diameters in normal pelves may be tabulated as follows: Interspinous—between the anterior superior iliac spines—25 cm. Intercristal—between the iliac crests—28 cm. External conjugate—taken from the upper border of the symphysis to the depression below the last lumbar vertebra—20.5 cm. Take 9.5 cm. from this to get the true conjugate. [Illustration: Fig. 93.—Various forms of pelvic deformity compared with the normal inlet. (Bumm.)] The circumference of the hips just below the iliac crests and above the trochanters—90 cm. It is taken with a tape line. These are the usual external measurements. The internal measurements are made with the fingers. [Illustration: Fig. 94.—The pelvimeter.] [Illustration: Fig. 95.—The various diameters of the inlet with the lengths given in cubic centimeters. (Williams.)] [Illustration: Fig. 96.—Measuring the distance between the anterior superior spines of the pelvis. (Williams.)] The diagonal conjugate is the distance from the lower border of the symphysis to the promontory of the sacrum. It should measure 12.5 cm. The first and second fingers are passed into the vagina and pushed up until the tip of the second finger touches the promontory of the sacrum. The finger of the other hand marks the depth of the examining fingers just below the symphysis. The distance is measured when the finger is withdrawn, and 1.5 cm. is subtracted. The result is the true conjugate. These measurements carefully made and the deduction judicially estimated, give one a fairly approximate idea of size and shape of the pelvic inlet. The aim of nearly all the pelvic measurements is to get not only the size and shape of the inlet, but so far as possible, a working estimate of the anteroposterior diameter of the brim, which is the most important of all the diameters. In normal cases this should be 11 cm. [Illustration: Fig. 97.—Measuring the external conjugate. (Williams.)] Thus, taking 9.5 cm. from the external conjugate (20.5 cm.) gives 11 cm. Subtracting 1.5 cm. from the diagonal conjugate as obtained with the fingers as above described, (12.5 cm.) gives 11 cm. The subtraction is made to compensate for the thickness of the pubic bone and its inclination outwards. [Illustration: Fig. 98.—Measuring the diagonal conjugate with the finger. (Eden.)] A circumference of 90 cm. corresponds to an inlet of 11 cm. in its anteroposterior diameter, and every variation of 5 cm. in this circumference makes a difference of 1 cm. (either larger or smaller) in the anteroposterior diameter.